By Paul Millman, PE, RA
SUPERSTRUCTURES Engineers + Architects Now that spring has arrived, it’s a good time for building owners and managers to assess the wear and tear on building façades due to the winter just past– and in particular to be aware of the effects of “frost jacking.”
In New York City, every time it rains or snows, water seeps into crevices, pores, and even micro-cracks in building façades and parapets, whether in concrete, brick and mortar, or other architectural materials. Unlike other liquids, water expands in size after it freezes. Cycles of precipitation at temperatures above freezing followed by temperatures below freezing expand the sizes of cracks, in a phenomenon known as “frost jacking,” eventually causing visible distress. This distress needs to be investigated to ensure that hazardous conditions haven’t occurred. As buildings age, these cracks can also become the source of water damage and even leaks into the interior of the building. In our region about two-thirds of the days below freezing are freeze/thaw days, and about one-third of the days below freezing are frost-jacking days, so a colder winter will produce proportionally more frost jacking days. Cold or warm, frost jacking days persist into March and even April. What is important with respect to potential damage to the building envelope is the accumulation of those days over time. SUPERSTRUCTURES’ website includes a Frost Jacking Day Tracker, which shows this accumulation – as of March 31st 2019 there had been 155 in the past 6 years. We’re using data provided to the National Weather Service by the weather station in Central Park. Since minimum and maximum temperatures are recorded at midnight, we look for days where the minimum temperature is 30 degrees F or lower preceded by days where the maximum temperature is 33 degrees F or higher, with rain and/or snow one or two days before. The cumulative number of frost jacking days isn’t a precise indicator of façade behavior: there are other variables, such as age, condition, orientation, materials of construction, etc. But it may help you understand why your building may be experiencing new problems and why it needs to be periodically inspected. Cycles of precipitation at temperatures above freezing followed by temperatures below freezing expand the sizes of cracks, in a phenomenon known as “frost jacking,” eventually causing visible distress.
GRAPHICS: SUPERSTRUCTURES Engineers + Architects SUPERSTRUCTURES’ Frost Jacking Day Tracker shows that as of March 31st 2019 there had been 155 such days in the past 6 years. (superstructures.com)